In most loudspeaker system for reproducing a larger part of the audio frequency spectra at least two drive units are used. An example being a woofer used for reproduction of sounds in the low frequency bands and a tweeter used for the high frequency bands. The voice coils of the separate drive units are via a cross-over filter network connected to a power amplifier, which provide the electrical signals representing the sound to be reproduced. The purpose of the cross-over filter is to provide each drive unit with electrical signals corresponding to the audio frequency range each drive unit is designed to reproduce. The characteristics of the filter are arranged so that around a cross-over frequency, in an intermediate band, the output to the woofer tails off with increasing frequency and the output to the tweeter tails off with decreasing frequency. The cross-over filter can for example be passive or active, digital or analogue. Careful matching of the characteristics of the filter with the characteristics of the drive units has to be undertaken to achieve good sound reproduction.
The loudspeaker system may incorporate more than two drive units. A three way system with a tweeter, a mid range woofer and a woofer is a common loudspeaker construction. The matching cross-over filter will divided the electrical signal to the drive units having to characteristic cross-over frequencies and two intermediate bands. The for the following discussion important observation, is that a loudspeaker system with more than one drive unit, will have a least one audio frequency band in which the sound is generated by more than one drive unit.
The sound radiated from each of the drive units may be said to emanate from the apparent sound source or acoustic center of that unit; the position of the acoustic center is a function of the design of the particular drive unit an may typically be determined by acoustic measurements. In addition may the absolute position of the acoustic center be dependent on the frequency of the emitted sound. When separate loudspeaker drive units are used, such as in the common two- and three-way systems briefly described above, the acoustic centers will be physically displaced from each other. The drive units are usually mounted on a common baffle such that their acoustic centers lie in a common plane, but they are offset in a vertical direction in the plane of the baffle. For a listener positioned approximately in line with the axes of the loudspeaker drive units and approximately equidistant from the acoustic centers of both drive units, a desired balance of output from the two drive units can be obtained. However, if the position of the listener is moved from the equidistant position, the distances between the listener and the acoustic centers of the loudspeaker drive units will be different and hence sounds in the intermediate frequency bands produced by two drive units, will be received by the listener with a difference in time. This time difference between sounds received results in a phase difference between the sounds received at the listening position. The sounds from the two drive units no longer add together as intended in the intermediate band or bands; the resultant received sound will be disordered.
An area of particular interest are Public Announcement (PA) in for example auditoriums and concert halls. Modem premises are often constructed in a way that the room itself is virtually acoustically mute. A suitable PA system typically comprises a number of high-Q loudspeakers (commonly high-Q homs) arranged so that, in principle, each listener has a free line of sight to a loudspeaker. This will limit, but not completely eliminate, the problems caused by the phase difference. An alternative approach is to have a large multitude of small loudspeakers operating at moderate acoustic levels, distributed close to the listener. More problematic is to amplify sound in acoustically complex, non-mute, often older premises such as churches, theaters and concert halls. These reverberant halls are often constructed to amplify the human voice or the sound of instruments by a multitude of reflections of the sound waves in walls and ceilings. If conventional loudspeakers, with a phase difference between the different drive units, are used in such an environment, each reflection will double the phase difference. When the sound, after a multitude of reflections, reaches the listener it will be highly distorted. To damp the hall to obtain a near acoustic mute environment is in most cases not an attractive solution, since the acoustic character of for example a church is perceived as an essential part of the sound experience of such a premises.
A number of attempts have been made to overcome the undesirable effects originating from the displacement of the acoustic centers of the drive units. It is known to combine the low and high frequency loudspeaker drive units in a single compound co-axial construction. The compound co-axial loudspeaker drive unit consists of a generally conical low frequency diaphragm driven by a voice coil interacting with a magnetic structure that has a central pole extending through the voice coil. A high frequency diaphragm is positioned to the rear of the structure and sound output from this diaphragm is directed to the front of the loudspeaker drive unit by means of a horn structure extending co-axially through the center pole of the magnetic structure which interacts with the low frequency diaphragm. Thus both the low frequency and high frequency sounds are directed in a generally forward direction from the compound loudspeaker drive unit. In this co-axial form of loudspeaker construction there is no vertical or horizontal offset of the apparent sound sources for low and high frequencies. However the low frequency diaphragm is positioned at the front of the loudspeaker unit whereas the high frequency diaphragm is positioned at the rear of the loudspeaker unit and this results in relative displacement of the acoustic centers in the direction of the axis of the drive unit causing an undesirable time difference in the arrival, at the listener, of sounds from the high and low frequency diaphragms. More recent attempts are taught in for example U.S. Pat. Nos. 4,492,826 and 4,552,242 in which at least one smaller speaker is mounted co-axially above the larger speaker. Both share, to a non neglectable degree, the drawback of the above-describe construction of having a relative displacement of the acoustic centers in the direction of the axis of the drive unit.
A compound loudspeaker drive unit with a low frequency unit and a high frequency unit with their acoustic center coinciding in all three dimensions is described in U.S. Pat. No. 5,548,657 and is commercially available. A miniature, but of conventional type, tweeter has been provided in a recess provided in the center pole piece of the woofer. Due to the miniaturization of the tweeter its efficiency will constitute a limitation. (Complex and costly methods of cooling, for example with ferrofluids, will be necessary in order to achieve an acceptable level of efficiency.) Although superior to previously described constructions, also this compound loudspeaker shows a phase difference that makes it less suitable for use in a multiple reflection environment. In addition, the teaching of U.S. Pat. No. 5,548,657, is limited to a compound loudspeaker that has two drive units, and is not applicable if three or more drive units are required.
Thus, there is a need in the art for providing an electro acoustic converter providing a coherent wave-front for the emitted sound waves in a full frequency range, needed for accurate sound reproduction in multi-reflectional environments, and still have a high power efficiency. High power efficiency typically anticipates efficient cooling of the voice coils and permanent magnets.